The invention relates to a method for detecting objects embedded in building structure subgrades such as cable, concrete reinforcing bars, conduit pipe and the like by sweeping or scanning a specified surface area of the building structure subgrade being analyzed by means of a hand-operated sensor with adaptive detection threshold. The invention also relates to a hand-operated sensor with an adaptive detection threshold for performing the method.
Inductive sensors, particularly eddy-current sensors, flow management detectors, magnetic sensors such as Hall, fluxgate and magnetoresistive sensors, but also capacitive detectors (e.g., stud sensors) or even sensors that measure an electrostatic field, such as hum detectors or live-wire detectors are possibilities for detectors in hand-operated or hand-held sensors for detecting objects embedded in normal building structure subgrades such as concrete, brick, wood, plaster, etc.
Objects in the sense meant here are principally elements embedded in walls and floors or similar subgrades, such as cable, concrete reinforcing bars, conduit pipe, undesired wood beams and the like, which must be detected and avoided as much as possible when processing the subgrade in question by means of digging equipment (drill, jackhammer, chisel and the like).
Because the signal strengths of typical hand-operated sensors, also designated as “field sensors,” depend very heavily on the surface distance of the aforementioned type of objects embedded in the building structure subgrade, namely up to the inverse sixth distance power, these types of sensors require very high signal dynamics in order to be able to detect objects close to the surface, on the one hand, and objects located deeper below, on the other. Detection by means of such a hand-operated sensor normally takes place by using a sensor to scan or sweep the location where a respective objectionable object is suspected to be several times. When using a threshold detector, starting at a specific signal strength it indicates that an object is located beneath the sensor. Because of influences from the subgrade itself, e.g., in the case of ferritic subgrades likes those represented by certain types of brick or magnetic pebble inclusions in concrete as well as due to altered dielectric properties, the conductivity of the subgrade and the like, the signal of the sensor is weakened more or less in an uncontrollable manner. A constant scanning threshold, i.e., a detector with a constantly set threshold value, cannot be used. In addition, the use of the sensor requires that even smaller objects and those located deeper below can still be detected reliably along with larger objects or those located closer to the surface, which supply a signal that is stronger by orders of magnitude. For this reason, adaptive detection thresholds are typically used in sensors of the aforementioned type.
In order that these types of adaptive detection thresholds function reliably requires that the entire to-be-analyzed surface area of the building structure subgrade be scrutinized at least once, i.e., swept or scanned by the hand-operated sensor. The detection threshold cannot be reliably set until a maximum has been passed through. If an object is not completely scanned in the process, then misdetections arise, which are also called “over-detections.” In this case, the sensor cannot differentiate whether the registered maximum was caused by scanning above and over the object or merely by approaching and changing direction with respect to the embedded object.
This problem is explained in greater detail on the basis of FIGS. 3a through 3c of the enclosed drawings:
A to-be-analyzed building structure subgrade 1 is scanned by means of a hand-operated sensor 10 (see FIG. 1, explained in greater detail in the following) in the region of a surface area 2 in a scanning direction x. Located beneath the surface at a specific distance is a to-be-detected object 3, for example a concrete reinforcing bar. A scan process with a sensor 10, which can also be designated as a sweep sensor, is explained based on Case A in FIG. 3a. The user starts the scan process to the left of the object 3. Since it is not known at the beginning how great the signal s(t) will be, the detection threshold for object detection is likewise unknown. A very low detection threshold is set initially, which is already exceeded at Point a (lower left diagram, FIG. 3a) so that in this case object indication is already activated. Once the object or the maximum of s(t) is scanned at Point b, the variable upper detection threshold can be set and object indication can be deactivated again. For precision localization, the user must now move the sensor 10 back so that the object detection will again be activated at the now known upper detection threshold at Point b and deactivated again at Point c.
The object indication range is now restricted as compared to the initial object scanning. Even if the user moves the sensor 10 back and forth several times, the upper detection threshold and thus the curtailed object indication range are maintained. In Case B, the sensor 10 initially approaches the object 3, but is moved backward before it reaches the object 3. In this case as well, an object is indicated between Points a and b during the first approach, and when moving back between Points b and c. However, in this case the object indication is inaccurate. In this context, this can be called an over-detection. The two Cases A and B cannot be differentiated in the time progression or signal progression s(t). In both cases, a specific detection threshold 5 is established and indicated as the case may be, whereby, however, Case B on the right represents a so-called “over-detection” within the meaning of the terminology used here.
FIG. 3b depicts another critical case. Here, the sensor 10 scans two embedded objects 31, 32 in Case A. According to FIG. 3a, when exceeding a first threshold at Point a, the object detection is activated, and deactivated again at Point b. The upper detection threshold is therefore known at Point b. However, it must be deleted again at Point d after scanning the local minimum, because it is not known how large the subsequent maximum will be. A new minimum threshold is set again at Point d, which is exceeded there so that the object indication is activated at d. The new upper detection threshold is not known until Point e. Here the object indication is deactivated again. By moving the sensor 10 back, the object indication range is restricted to Points e and f according to FIG. 3a. In Case B in FIG. 3b, the sensor is moved back and forth over the object 3 as in the case in FIG. 3a. The same signal signature s(t) as in Case A in FIG. 3a emerges. At Point d, however, in contrast to Case A, the upper detection threshold determined at Point b may not be deleted so that the restricted object indication range is retained.
FIG. 3c shows the behavior according to FIG. 3b for a differential sensor. The sensor 10 scans two embedded objects 31, 32 in Case A. In the case of the differential sensor, the object positions correspond to the zero passages of the negative slope of the signal s(t). Located precisely between the two objects 31, 32 for reasons of symmetry is another zero passage b with a positive slope, which is not connected to an object 3 however. This produces an inaccurate object indication or over-detection between the two objects. In Case B, the sensor is again moved back and forth over an object.
In this case, the zero passage b with a positive slope corresponds very well to the object position.
With all three cases explained on the basis of FIGS. 3a through 3c, a clear allocation of a buried object is not reliably possible with the previously known sweep sensor, or with the differential sensor, because a differentiation cannot be made between the respective Cases A and B.
The objective of the invention is avoiding an over-detection or at least reducing an over-detection rate of the sensors of the aforementioned type in order to thereby increase the reliability of object detection. In addition, efforts are also made to improve the precision and reliability in determining an object over-detection, in particular concrete reinforcing bar over-detection and the diameter of the concrete reinforcing bars concealed in the building structure subgrade.
The invention relates to a method for the detection of objects embedded in building structure subgrades such as cable, concrete reinforcing bars, conduit pipes and the like by sweeping a hand-operated sensor with an adaptive detection threshold over a specified surface area of the building structure subgrade being analyzed, wherein to reduce an over-detection with the adaptation of the detection threshold, a change in the direction of movement of the sensor being guided over the surface area is recorded with respect to an embedded object. In particular, the acceleration of the hand-operated sensor is recorded in at least two directions with respect to an embedded object in order to compute the corresponding speeds at least as estimates from the acceleration values so that a conclusion about a reversal of the direction of movement of the hand-operated sensor can be determined with respect to an embedded object. A signal is advantageously generated as a function of the sensor path from the estimated value(s) of the speed, correlated with the temporal progression of the signal supplied by the object and a conclusion about the dimensions of the embedded object is obtained therefrom.
A hand-operated sensor with adaptive detection threshold for the detection of objects embedded in building structure subgrades such as cable, concrete reinforcing bars, conduit pipes and the like by sweeping a surface area of a building structure subgrade being analyzed is characterized according to the invention in that the sensor for reducing an over-detection with the adaptation of the detection threshold is equipped with a motion sensor device for determining a change in the direction of movement of the sensor guided over the surface area with respect to an embedded object. It is preferred that the motion sensor device is equipped with an at least biaxial acceleration sensor and a device fed by the signals of the acceleration sensor for measuring the momentary sweeping speed of the sensor and for recording the directional change points of the sensor sweeping over the surface area with respect to an embedded object. As a rule and preferably, the device for measuring the momentary sweeping speed is equipped with at least one integrator for each acceleration sensor for determining the momentary sweeping speed of the sensor and for detecting a directional change with respect to an object embedded in the building structure subgrade.
Moreover, it is advantageous to equip the sensor with a device for recording the path covered by the sensor when scanning the surface area by sweeping, wherein the device for recording the path has an optical, two-dimensional correlator for determining the relative movement of the to-be-analyzed surface area of the building structure subgrade. In this case, an optical visualization device can also be provided, on which a representation of the trajectory of the sensor sweeping over the surface area is displayed based on the signals supplied by the correlator. The correlator can have at least two laser distance sensors at a predetermined angle from each other, or, alternatively, a laser-stream sensor device integrated into the sensor and functioning in the manner of a computer mouse.
In the case of an advantageous embodiment of the invention optimized for determining the detection threshold of the sensor, a triplet of acceleration sensors is provided, on the one hand, with a pair of acceleration sensors that are at right angles to one another as well as a third acceleration sensor arranged at a fixed distance from the acceleration sensor pair, whose measuring axis is aligned with one of the measuring axes of the acceleration sensor pair that are at right angles to one another. In order to also be able to record the rotational angle of the sensor around a fixed point, a gyroscope can also be integrated into the sensor, wherein the acceleration sensors can be micromechanical acceleration indicators, on the one hand, and the gyroscope can advantageously be a micromechanical vibration rotational rate sensor according to the Coriolis principle.
The invention and advantageous details are explained in greater detail in the following in examples of exemplary embodiments making reference to the drawings.